8 FOREST TREE DISEASES. 



a great number of needles are destroyed the regular 

 life functions of the tree are affected and it becomes 

 diseased. With trees, as with animals, the various life 

 functions are confined to certain organs, and anything 

 which affects the health of these affects the functions an 

 well. Disease is an unbalancing of normal functions, 

 and to understand its nature we must first understand 

 these functions and their relations to each other. 



STRUCTURE OF THE TREE. 



Trees grow in length in the youngest (end) part of 

 the twigs and at the very ends of the roots; they grow 

 in thickness in the so-called cambium, a very thin layer 

 of cells between the bark and the wood of roots, stem, 

 and branches. The cells of the twig ends, of the root 

 tips, and of the cambium are capable of division, and 

 certain young cells produced by this division, if properly 

 nourished, grow to normal size, divide in turn, and so 

 on. The growth in thickness (diameter) by the cam- 

 bium takes place only in spring and early summer, and 

 results in the production of a thin layer of wood all 

 over the tree. In cross section this layer appears in the 

 shape of the so-called annual ring, corresponding to one 

 year's growth in diameter. The springwood of the an- 

 nual ring is light and .soft; the summerwood appears 

 as a narrower, darker line and is harder than the other. 



After a number of years the cells of the oldest an- 

 nual rings of wood die and, after undergoing chemical 

 changes, form what is called the heartwood. The liv- 

 ing wood outside of the heartwood is called sapwood. 



